This invention relates to a method of growing epitaxial layers using a magnetron sputter source in molecular beam epitaxy apparatus.
Magnetron sputter epitaxy has become increasingly popular as a technique for depositing thin layers. For example, in recent years there has been tremendous interest in GaN based III-N materials. A dramatic improvement in the material quality has led to the development of high brightness light emitting diodes, and more recently xe2x80x9cbluexe2x80x9d laser diodes. There has also been a dramatic improvement in the performance of high power microwave metal-semiconductor field-effect transistors and modulation doped field-effect transistors based on these materials.
Crucial to all these applications is the growth of material with high crystalline quality and of high purity. Magnetron sputter epitaxy (MSE) is a technique that has been used to grow GaN. Typically GaN epilayers are grown on sapphire substrates, which are highly lattice mismatched, necessitating the predeposition of a thin (xcx9c500 xc3x85) buffer/nucleation layer of either GaN or AlN. The observed electrical and optical properties of the resulting GaN layers is strongly dependent on the dislocation density and of the overall impurity content.
Prior art magnetron sputter sources require special MBE machines. An object of the invention is provide an MSE source that will fit onto a standard MBE machine.
According to the present invention there is provided a method wherein a magnetron sputter source is adapted to fit into a K-cell port in a molecular beam epitaxy apparatus, the magnetron sputter source comprising a protruding cylindrical body for insertion into the K-cell port, said cylindrical body being attached at its proximal end to a flange and having its distal end open; an array of permanent magnets arranged at the distal end of said cylindrical body; a magnet return piece mounted behind said permanent magnets; a sputter target mounted in front of said permanent magnets; and cooling ducts within said cylindrical body for carrying a cooling medium to said sputter target.
The magnetron sputter source can be used for depositing gallium nitride or like epilayers as disclosed in U.S. Pat. No. 6,291,318. Using this method, silicon doped GaN epilayers having room temperature electron mobilities  greater than 550 cm2/V s can be grown on (0001) sapphire. Unlike other growth techniques, the initial buffer/nucleation layer, preferably of AlN (aluminum nitride), is grown by MSE. The magnetron sputter source in accordance with the invention can also be used for other applications.
The magnetron sputter source can be used in a dual mode MBE/MSE system. The MSE technique differs from conventional MBE in that an ultrahigh vacuum dc magnetron sputter cathode is used as the group III source and deposition of the layers occurs in the pressure range of 1-5 mTorr.
Typically, the MSE technique is employed only for the growth of a buffer/nucleation layer. The GaN layer is deposited by ammonia MBE where a conventional dual filament K cell is used for the gallium source, and high purity ammonia is used as the source of nitrogen.
Preferably, the deposition system is equipped with a substrate holder capable of heating the 2 in. sapphire(0001) wafers to temperatures in excess of 1000xc2x0 C. Typical growth temperatures for the GaN layers were in the range of 860-920xc2x0 C. as measured by an optical pyrometer (emissivity set to 0.3).